The field of the invention relates generally to turbine engines, and more specifically to film cooling of turbine engines.
In a gas turbine engine, air pressurized in a compressor is mixed with fuel in a combustor to generate hot combustion gases. Energy is initially extracted from the gases in a high pressure turbine (HPT) that powers the compressor, and subsequently in a low pressure turbine (LPT) that powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and/or industrial applications.
Generally, engine efficiency increases as the temperature of combustion gases is increased, but the increased gas temperature increases the operating temperature of various components along the gas flowpath, which in turn increases the need for cooling such components to facilitate extending their useful life.
For example, known combustors include outer and inner liners which require cooling during operation. Known turbine nozzles include hollow vanes which also require cooling. In at least some turbine engines, flowpath components exposed to hot combustion gases are cooled using compressor bleed air, which subsequently reduces engine efficiency since the bled air is not used in the combustion process. For example, at least some known components channel the compressor bleed air through film cooling holes.
At least some known cooling holes are formed from a cylindrical bore that is oriented at a shallow angle through the heated wall to enable a film of cooling air to be discharged along the external surface of the wall. Discharging the air at a shallow angle reduces the likelihood of undesirable blow-off and/or flow separation. The amount of surface area to be film cooled is typically only increased by increasing the number of cooling holes and thus increases the amount of air discharged therefrom. However, increasing the amount of cooling air decreases engine efficiency.
To improve the efficiency of known cooling holes, at least some cooling holes are formed with a divergent discharge end to diffuse the cooling air as it is discharged from the cooling hole outlet.
However, diffusion in film cooling holes may be limited due to the half-angle of the diffusion outlet to prevent flow separation. For example, within known cooling holes, the diffusion angle may be limited to about ten degrees on each side of the outlet to prevent overexpansion of the discharge cooling air which could lead to undesirable film separation.
Accordingly, it is desired to provide an improved film cooling hole that can produce increased film coverage without increasing the amount of cooling air required and without increasing the likelihood of flow separation of the film cooling air.